Polyethylene filaments, films and tapes are well known in the art. However, until recently, the tensile properties of such products have been generally unremarkable as compared to competitive materials such as polyamides and polyethylene terephthalate.
In recent years, many processes for the preparation of high tenacity filaments and films of high molecular weight polyolefins have been described. The present invention is an improvement of the processes and products described in U.S. Pat. Nos. 4,413,110, 4,663,101, 5,578,374, 5,736,244 and 5,741,451, each herein incorporated by reference in their respective entireties. Other processes are known and have been used to prepare single filaments of exceptionally high strength and modulus. For example, A. V. Savitski et. al. In Polymer Science U.S.S.R., 26, No. 9, 2007 (1984) report preparing a single polyethylene filament of 7.0 GPa (81.8 g/d) strength. In Japanese patent JP-A-59/216913 a single filament of 216 GPa (2524 g/d) modulus is reported. However, as is well known in the fiber spinning arts, the difficulty of producing strong yarns increases with increasing numbers of filaments.
It is an object of this invention to provide high tenacity, high modulus polyethylene multi-filament yarns having a unique and novel microstructure and very high toughness. Such multi-filament yarns are exceptionally efficient in absorbing the energy of a projectile in anti-ballistic composites.
Other objects of this invention along with its advantages will become apparent from the following description.
The present invention is directed to a method of preparing a high tenacity, high modulus multi-filament yarn comprising the steps of: extruding a solution of polyethylene and solvent having an intrinsic viscosity (measured in decalin at 135xc2x0 C.) between about 4 dl/g and 40 dl/g through a multiple orifice spinneret into a cross-flow gas stream to form a fluid product; stretching the fluid product (above the temperature at which a gel will form) at a stretch ratio of at least 5:1 over a length of less than about 25, mm with the cross-flow gas stream velocity at less than about 3 m/min; quenching the fluid product in a quench bath consisting of an immiscible liquid to form a gel product, stretching the gel product; removing the solvent from the gel product to form a xerogel product substantially free of solvent; and stretching the xerogel product, with a total stretch ratio sufficient to product a polyethylene multi-filament yarn characterized by a tenacity of at least 35 g/d, a modulus of at least 1600 g/d, and a work-to-break of at least 65 J/g.
The method further comprises the step of stretching the fluid product at an extension rate of more than about 500 minxe2x88x921.
The extruding step preferably is carried out with a multi-orifice spinneret wherein each orifice possesses a tapered entry region followed by a region of constant cross-section and wherein the ratio of the length/transverse dimension is greater than about 10:1. Further, the length/transverse dimension may be greater than about 25:1.
The present invention further includes a polyethylene multi-filament yarn of about 12 to about 1200 filaments having a denier of about 0.5 to about 3 denier per filament (dpf), a yarn tenacity of at least about 35 g/d, a modulus of at least 1600 g/d, and a work-to-break of at least about 65 J/g. The microstructure of the multi-filament yarn contains a high strain orthorhombic crystalline component comprising more than about 60% of the orthorhombic crystalline component and it may have a monoclinic crystalline component greater than about 2% of the crystalline content. In a preferred embodiment, the yarn includes about 60 to about 480 polyethylene filaments having a denier of about 0.7 to about 2 dpf, a yarn tenacity of about 45 g/d, a modulus of about 2200 g/d, greater than about 60% of a high strain orthorhombic crystalline component, and a monoclinic crystalline component greater than about 2% of the crystalline content.
The present invention also includes a composite panel comprising a polyethylene multi-filament yarn having a tenacity of at least about 35 g/d, a modulus of at least 1600 g/d, a work-to-break of at least about 65 J/g wherein the yarn has greater than about 60% of a high strain orthorhombic crystalline component and the yarn has a monoclinic crystalline component greater than about 2% of the crystaline content.
The present invention further includes a ballistic resistant composite panel having an specific energy absorption of the composite (SEAC) of at least about 300 J-m2/Kg against .38 caliber bullets using test procedure NILECJ-STD-0101.01.